Ultrasonic transducer assemblies have found wide use in a number of measurement situations. The ultrasonic transducer itself is typically formed of an electroacoustical transducer producing bulk ultrasonic waves in response to controlled electrical signals. The transmission characteristics, either velocity or attenuation, of the ultrasonic waves through the medium being measured provide an indication of various key characteristics. One very common application is the application of ultrasonic signals to measure the flow of materials through pipes or other conduits. In this application it is often desirable to employ a clamp-on transducer which can be permanently or temporarily attached or coupled to the outer wall of the pipe. Depending in part upon the angle of incidence of the waves propagated from the electroacoustical transducer, the waves as generated may be used for measurement or alternatively Rayleigh, Lamb, or other waves generated by mode conversion at the pipe wall or the fluid pipe wall interface, may be employed.
A single transducer assembly can be used to Perform the measurement employing a technique involving reflection of the ultrasonic waves from a surface within the pipe and measuring the received reflected wave. Many times, however, in measurement of fluid characteristics within a pipe a pair of transducers are employed for generating a signal at one transducer and receiving it at the other. In some arrangements the transmitter and receiver functions of the pair of transducers are alternated in time.
In the use of clamp-on transducers on pipe walls where the pipe is at either very high temperature or low temperature, problems arise due to the limited temperature range over which the usual electroacoustical transducers can operate.
One previous solution to this problem was to use a LiNbO.sub.3 piezoelectric element as the transducer. However, this element requires an oxygen-bearing atmosphere at high temperature to avoid decomposing, and also suffers from differential expansion coefficients along different axes. In a number of transducer systems it has been conventional to use a buffer element providing a solid material forming an acoustic transmission path between the electroacoustic transducer and the surface of the pipe or other material into which the acoustic wave is to be transmitted. Requirements of acoustic transmission characteristics and strength have dictated the choice of these materials, which, for the most part, have been metallic. Such buffers are, of course, good heat conductors and hence do not serve adequately to isolate the electroacoustic element from high temperature surfaces when these are the material to be contacted for measurement. In addition metallic buffers often do not provide acoustical properties (low sound speed, moderately high attenuation) that are particularly desirable in a buffer especially where certain mode conversions in the pipe or other adjacent solid medium are desired.